HTTP error 413 is supposed to occur when the size of the data you are uploading is too big. It would be bad practice (though that hasn't ever stopped anybody) for them to give this error when the image contained in that data has too big a graphical size. The upload I'm assuming you're referring to has a size of 8.2 MiB. FA's limit is 10 MiB. Scaling this size up by (3/2)^2 (which is somewhat simplistic but should work well enough) gets the full image at about 18 MiB, which is above that upload limit.

It looks like the paintbrush strokes and the jagged lines is what made it complex enough to get above the limit.

Re-compressing this PNG that you have uploaded with the GIMP gets it down to 5.3 MiB, which makes me think that Photoshop or wherever it was originally from was not set for maximum (level 9) PNG compression. But (going by the scaling), this is still not enough. I'd say you have 3 options to try to upload this or similar images in the future:

• Set the compression level to 9 and get lucky.
• Play with the dimensions (at level 9) until you get the largest you can that's under 10 MiB.
• Save it as a JPEG instead (even at level 100, it fits into the FA limit, assuming that my scaling factor works perfectly). This is probably the best option in the case of something like this.
(An extreme solution would be to use something like PNGCrush, but as I understand it it takes something on the order of hours to run.)

If I could save a comment I’d do that in a heartbeat. Thank you so much for this info! I have to try something like this in the future.